r - Select cases in a data frame -

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i want select range of cases in data frame, every n column. illustrate problem reproducible example:

set.seed(100) data <- data.frame(replicate(18,sample(0:100,18,rep=true)))

from data, want select data[1:6, 1], data[7:12, 7], data[13:19, 13], , forth. obviously, i'm working bigger dataset (>10000 rows , columns), why prefer automated way this.
have tried define sequence beforehand (seq()) couldn't quite figure out how apply problem.
help!

matrix indexing might handy here:

sel <- cbind(sequence(nrow(data)),rep(seq(1,ncol(data),6),each=6)) sel  #      row  col #      [,1] [,2] # [1,]    1    1 # ... # [6,]    6    1 # [7,]    7    7 # ... #[12,]   12    7 #[13,]   13   13 # ...

then:

data[sel] # [1] 31 26 55  5 47 48 97  3 92 73 20 84 37 30 55 37 85 62

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